Imaging antiferromagnetic domains in LiCoPO$_4$ via the optical magnetoelectric effect

Antiferromagnetic (AFM) materials are considered as promising building blocks of novel data storage devices, still, detecting and manipulating AFM domains have remained challenging. Here, we demonstrate that the two antiphase domains of the magnetoelectric antiferromagnet LiCoPO$_4$ can be distinguished by their light absorption difference. Using visible and infrared spectroscopy, we observed spontaneous non-reciprocal absorption, also termed as directional dichroism, at the crystal field excitations of Co$^{2+}$ ions coordinated by distorted oxygen octahedra. This absorption contrast is particularly pronounced near the telecommunication wavelength of 1550 nm. These findings allowed us to image the AFM domains in LiCoPO$_4$ using a simple transmission light microscopy setup. Our findings suggest that optical magnetoelectric effects offer promising routes for probing the AFM order parameter in non-centrosymmetric transition metal compounds.